x86/cpufeatures: Add Slow Memory Bandwidth Allocation feature flag

Add the new AMD feature X86_FEATURE_SMBA. With it, the QOS enforcement policies
can be applied to external slow memory connected to the host. QOS enforcement is
accomplished by assigning a Class Of Service (COS) to a processor and specifying
allocations or limits for that COS for each resource to be allocated.

This feature is identified by the CPUID function 0x8000_0020_EBX_x0[2]:
L3SBE - L3 external slow memory bandwidth enforcement.

CXL.memory is the only supported "slow" memory device. With SMBA, the hardware
enables bandwidth allocation on the slow memory devices.  If there are multiple
slow memory devices in the system, then the throttling logic groups all the slow
sources together and applies the limit on them as a whole.

The presence of the SMBA feature (with CXL.memory) is independent of whether
slow memory device is actually present in the system. If there is no slow memory
in the system, then setting a SMBA limit will have no impact on the performance
of the system.

Presence of CXL memory can be identified by the numactl command:

  $numactl -H
  available: 2 nodes (0-1)
  node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
  node 0 size: 63678 MB node 0 free: 59542 MB
  node 1 cpus:
  node 1 size: 16122 MB
  node 1 free: 15627 MB
  node distances:
  node   0   1
     0:  10  50
     1:  50  10

CPU list for CXL memory will be empty. The cpu-cxl node distance is greater than
cpu-to-cpu distances. Node 1 has the CXL memory in this case. CXL memory can
also be identified using ACPI SRAT table and memory maps.

Feature description is available in the specification, "AMD64 Technology
Platform Quality of Service Extensions, Revision: 1.03 Publication # 56375
Revision: 1.03 Issue Date: February 2022" at
https://bugzilla.kernel.org/attachment.cgi?id=301365

See also https://www.amd.com/en/support/tech-docs/amd64-technology-platform-quality-service-extensionsSigned-off-by: Babu Moger <babu.moger@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Link: https://lore.kernel.org/r/20230113152039.770054-3-babu.moger@amd.com

arch/x86/include/asm/cpufeatures.h

@@ -307,6 +307,7 @@
 #define X86_FEATURE_SGX_EDECCSSA	(11*32+18) /* "" SGX EDECCSSA user leaf function */
 #define X86_FEATURE_CALL_DEPTH		(11*32+19) /* "" Call depth tracking for RSB stuffing */
 #define X86_FEATURE_MSR_TSX_CTRL	(11*32+20) /* "" MSR IA32_TSX_CTRL (Intel) implemented */
+#define X86_FEATURE_SMBA		(11*32+21) /* "" Slow Memory Bandwidth Allocation */
 
 /* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */
 #define X86_FEATURE_AVX_VNNI		(12*32+ 4) /* AVX VNNI instructions */

arch/x86/kernel/cpu/scattered.c

@@ -45,6 +45,7 @@ static const struct cpuid_bit cpuid_bits[] = {
 	{ X86_FEATURE_CPB,		CPUID_EDX,  9, 0x80000007, 0 },
 	{ X86_FEATURE_PROC_FEEDBACK,    CPUID_EDX, 11, 0x80000007, 0 },
 	{ X86_FEATURE_MBA,		CPUID_EBX,  6, 0x80000008, 0 },
+	{ X86_FEATURE_SMBA,		CPUID_EBX,  2, 0x80000020, 0 },
 	{ X86_FEATURE_PERFMON_V2,	CPUID_EAX,  0, 0x80000022, 0 },
 	{ X86_FEATURE_AMD_LBR_V2,	CPUID_EAX,  1, 0x80000022, 0 },
 	{ 0, 0, 0, 0, 0 }